1. Field of the Invention
This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2007-120298, filed on Apr. 27, 2007, the disclosure of which is incorporated herein in its entirety by reference.
The present invention relates to a radio communications system having a plurality of radio zones (hereinafter, referred to as cells) and, more particularly, to a method and a device for controlling the transmission power of an uplink access control signal.
2. Description of the Related Art
In a mobile communications system, for a base station and a mobile station to perform data communication, they need to establish synchronization between them in advance. Since initial access from the mobile station in particular is not always in synchronization, the base station requires some procedure for uplink synchronization with the mobile station.
According to Long Term Evolution (LTE), which is being standardized by the 3rd generation partnership project (3GPP), a random access channel (RACH) and an uplink shared channel (UL-SCH) are provided for uplink synchronization and uplink data transmission. The RACH is a channel to transmit a control signal for the establishment of uplink synchronization and further to request a resource for the transmission of uplink data. To establish uplink synchronization without a long delay, it is preferable that the probability of RACH transmission collision be reduced as low as possible (see 3GPP TS 36.300 V.1.0.0, Mar. 19, 2007). On the other hand, the UL-SCH is a channel to transmit data and Layer-2/Layer-3 control packets.
FIG. 1A is a diagram schematically showing a generic structure of a mobile communications system according to LTE, and FIG. 1B is a resource structure diagram schematically showing radio resources based on both frequency-division and time-division techniques.
In wideband code division multiple access (WCDMA), a RACH and an enhanced dedicated channel (EDCH) share the same frequency resource, multiplexed by using different spreading and scrambling codes. On the other hand, in LTE, a plurality of frequency-divided and time-divided resources are shared by a RACH and an UL-SCH exclusively of each other. Specifically, the LTE uplink has a resource structure in which a system bandwidth of 10 MHz is time-divided into time intervals of 1 msec, each of which is further frequency-divided into widths of 1.25 MHz. Referring to FIG. 1B, each of t1, t2, . . . on the horizontal axis corresponds to a 1-msec-long time resource, and each of FB#1, FB#2, . . . on the vertical axis corresponds to a 1.25-MHz-wide frequency resource. Hereinafter, one rectangular block defined by one time resource and one frequency resource as shown in FIG. 1B will be simply referred to as “resource.”
It is each base station eNB that determines how to allocate such system resources to the RACH and UL-SCH. In general, a RACH resource is periodically allocated as shown in FIG. 1B so that a mobile station UE can gain access to the base station without a long delay. It is also possible to allocate a plurality of RACH resources at a time and thereby secure a sufficient RACH access capacity. Each base station eNB generally broadcasts information indicative of which resources) is allocated to the RACH Therefore, every mobile station UE (User Equipment) in a cell can gain access to the RACH resource(s) whenever the mobile station UE needs, in accordance with the broadcast information.
In LTE, some were of the opinion that there was no need to control RACH transmission power, because it had been decided at the beginning that the same frequency band was not shared between the RACH and UL-SCH. In the case where the RACH and UL-SCH are frequency-divided, even if a mobile station UE performs RACH transmission at maximum transmission power, no interference occurs with the UL-SCH transmission of another mobile station UE. Accordingly, there is no significant reason to control RACH transmission power at least within a single cell, from the viewpoint of the occurrence of interference.
However, in the case where a plurality of resources are shared by the RACH and UL-SCH exclusively of each other as shown in FIG. 1B, and where resource allocation is determined in each cell, there is a possibility that the same resource that is allocated for RACH transmission in one of neighboring cells may be allocated for UL-SCH transmission or RACH transmission in the other cell. Accordingly, when a mobile station UE is performing RACH transmission to its serving cell at maximum transmission power, the possibility cannot be ignored that this RACH transmission will interfere with UL-SCH transmission or RACH transmission performed in a neighboring cell. To prevent such inter-cell interference, it is preferable to control RACH transmission power by using some method.
For one of the methods for reducing inter-cell interference as much as possible, power ramping control has been proposed (see 3GPP TS 36.300 V.1.0.0, Mar. 19, 2007). Hereinafter, RACH transmission power control through power ramping will be described briefly.
FIG. 2A is a sequence diagram showing a procedure of uplink access through a RACH, and FIG. 2B is a schematic time chart showing an example of the power ramping performed before synchronization is established through a RACH. For example, when a mobile station UE desires to transmit data, the mobile station UE first receives a broadcast channel (BCH) broadcast by a base station eNB, thereby acquiring an initial coefficient K and RACH resource information. The mobile station UE then calculates initial RACH transmission power Pinit by using a path loss PLOSS, which is measured from a pilot signal (reference signal), and the initial coefficient K (for example, Pinit=K×PLOSS). Thus, the initial RACH transmission power Pinit is set so that the reception quality of a RACH signal received by the base station eNB will be kept at a desired level. The mobile station UE performs RACH transmission using a RACH resource at this initial transmission power Pinit. Upon arrival of this RACH transmission at the base station eNB, the base station eNB sends a RACH response including a value for timing adjustment back to the mobile station UE, whereby the mobile station UE can establish physical-layer synchronization. After the establishment of synchronization, the mobile station UE sends a scheduling request to the base station eNB. The base station eNB allocates UL-SCH resources in response to this request and sends back a transmission grant. Thus, the mobile station UE transmits data packets by using the allocated UL-SCH resources.
However, when a RACH signal does not arrive at the base station eNB due to an increase in the level of interference or due to the occurrence of fading, the mobile station UE receives no RACH response even after a predetermined period of time has passed. Therefore, the mobile station UE increases its RACH transmission power by a predetermined step Pdel at each time as shown in FIG. 2B until the mobile station UE receives a RACH response. Such increasing in RACH transmission power step by step is called “power ramping”. A period during which the RACH transmission power is increased step by step, starting from the first RACH transmission, is called “power ramping period,” and this power ramping period plus an elapsed time before physical-layer synchronization is established upon receipt of the RACH response is called “RACH access delay.” Accordingly, when a RACH transmission collision occurs, the RACH access delay increases.
Moreover, the sum of the RACH access delay and an elapsed time between the establishment of physical-layer synchronization and the establishment of connection upon receipt of the transmission grant is called “LTE call setup delay.” The LTE call setup delay also serves as a key performance indicator (KPI), an object of evaluation, of system performance.
By RACH transmission power control through the power ramping, the RACH transmission power of the mobile station UE can be set at such a level that sufficient RACH reception quality is secured at the base station eNB while interference to a neighboring cell is suppressed. For example, in the case of a mobile station UE having a low path loss, since power ramping can be started from a lower power level than the initial RACH transmission power Pinit, the mobile station UE can establish synchronization by RACH transmission at lower transmission power than a mobile station UE having a high path loss. Accordingly, uplink interference to a neighboring cell can be effectively prevented.
However, according to the above-described power ramping control, once RACH interference occurs between neighboring cells, a phenomenon called “party effect,” which will be described below, may be caused, in which the initial RACH transmission power increases in all cells one after another.
FIG. 3A is a diagram schematically showing how neighboring cells interfere with each other, FIG. 3B is a graph showing the RACH access delay varying with the level of RACH interference, and FIG. 3C is a graph showing the RACH access delay varying with the load of RACH access. Moreover, FIG. 4 is a time chart showing variation in the average RACH access delay and variation in the RACH transmission power offsets to describe the party effect between neighboring cells.
Generally, a base station tries to make the RACH access delay as short as possible because failing in gaining RACH access causes a delay in a call setup or handover procedure. The RACH access delay can be reduced by increasing the initial RACH transmission power as described above. Here, referring to FIG. 3A, it is assumed that the initial RACH transmission power Pinit has been increased in a cell A. This increase in initial RACH transmission power in the cell A causes an increase in RACH interference with a neighboring cell B. When the RACH interference is increased, a base station eNB2 controlling the cell B allows the initial RACH transmission power Pinit to be increased in the cell B in order to shorten the RACH access delay. This increase in initial RACH transmission power in the cell B causes an increase in interference with the neighboring cell A and another neighboring cell C. In each of the cells A and C, since the RACH interference is increased, the initial RACH transmission power is further increased. In this manner, a phenomenon called “party effect” is caused, in which the cause and effect repeat between neighboring cells, whereby the initial RACH transmission power is increased more and more. Accordingly, once interference occurs, the initial RACH transmission power is reciprocally increased between neighboring cells, which causes a chain reaction, resulting in the initial RACH transmission power Pinit being ultimately set at a maximum value in every cell.
Referring to FIGS. 3B and 3C, in general, as the level of RACH interference becomes higher, or as the frequency of RACH access becomes higher (the load becomes larger), the RACH access delay becomes longer. The level of RACH interference can be measured, for example, as the number of failed RACH transmissions (that is, the number of RACH transmissions made before a RACH response shown in FIG. 2B is received), which is reported to a base station from a mobile station present in the cell of the base station. Therefore, the RACH access delay is measured by using this number, and statistical processing is further performed, whereby the average RACH access delay can be obtained. Moreover, the frequency of RACH access (the load of RACH access) can be measured as the number of times a base station receives RACH access from mobile stations present in the cell of the base station. Therefore, this number is similarly subjected to statistical processing, whereby the average RACH access delay can be obtained.
When the thus-measured RACH access delay increases, each base station eNB raises a RACH transmission power offset as shown in FIG. 4 so that the initial RACH transmission power will be increased step by step. This operation causes the above-described “party effect,” resulting in the initial RACH transmission power being set at a maximum value in every cell in the end. This is a cause for degradation of the uplink capacity in the entire network.
Such a problem concerns not only the LTE, but may exist in cell-based general radio communications systems using an access scheme (FTDMA) based on a frequency-divided and time-divided resource structure. Particularly in a system in which resource allocation control is individually performed by base stations or radio communication devices controlling respective cells, there is no provision of a mechanism of automatically controlling RACH transmission power in the entire network. Therefore, inter-cell interference cannot be controlled, leading to the easy occurrence of the above-described “party effect.”